Over the last few years, the crystallographic technique known as MAD, or
multiwavelength anomalous diffraction, has emerged as one of the more powerful
methodologies available to the macromolecular crystallographer. Obviously, a tunable
beam line with the usual high flux and low divergence is necessary to carry out such an
experiment.
At CHESS, such a facility exists at the F2 doubly-focused wiggler beam line. Station F2
accepts 2 mrad of radiation which is monochromatized by a fixed offset,
sagitally-focusing monochromator. A rhodium coated Si mirror located downstream of the
monochromator is used to focus the beam vertically and to suppress unwanted harmonics.
This doubly focused wiggler beam has a focal size of 3.6 mm x 0.32 mm in the hutch. At 13
keV, the flux is 1010 ph/s after a 0.3 mm collimator (at 100 mA).
Provisions have been built into the MAD setup which permit a rapid periodic check of the
calibration of the monochromator to insure that the anomalous signal is being properly
recorded. With one computer command, the experimenter rotates a reference foil into the
beam at the upstream end of the hutch and initiates a quick scan of the monochromator
through the edge energy. After the absorption spectrum is displayed on the terminal, the
reference foil and the monochromator return to their starting positions. If the spectrum
shows a shift in the apparent edge position, the monochromator adjusted to
restore proper calibration. This
process is typically carried out at the beginning of each fill, consuming less
than 1 minute of beam time for each check.
Schematic of the F2 hutch for MAD experiments. Note that the foil is moved into the beam
to perform quick energy calibration scans.
More details on performing MAD experiments at F2 are available.
2008 Run
Nov 19th - Dec 22nd
